Protonation of Naphthalene-(Water)n Nanoclusters: Intracluster Proton Transfer to Hydration Shell Revealed by Infrared Photodissociation Spectroscopy.

Solvation-dependent intracluster proton transfer (ICPT) within bare and Ar-tagged protonated naphthalene-(water)n clusters, H+(Np-Wn) with n ≤ 3, is characterized by infrared photodissociation (IRPD) spectroscopy in a supersonic plasma expansion. IRPD spectra of size-selected clusters recorded in the CH and OH stretch range (2750-3800 cm-1) are analyzed with dispersion-corrected density functional theory (DFT) calculations (B3LYP-D3/aug-cc-pVTZ) to determine both the protonation site and the structure of the hydration network. Ar tagging of H+(Np-Wn) leads to colder spectra with higher spectral resolution. The position of the excess proton is controlled by a subtle balance between the difference in proton affinity (PA) of Np and Wn and the involved solvation energies. For n = 1, the excess proton is localized on the Np ring, leading to a H+Np-W structure with a bifurcated CH···O ionic H-bond, because of the large difference in PA of Np and W. For n = 2, ICPT occurs, and the cluster has a structure in which a symmetric Zundel ion is connected to Np via two strong OH···π ionic H-bonds. Because of the similar PA values of W2 and Np, the energetics of the ICPT is largely decided by the higher solvation energy in favor of Np-H+W2 as compared to H+Np-W2. For n ≥ 3, the PA of Wn substantially exceeds the one of Np, leading to ICPT. Attachment of the bulky planar Np ring to H+Wn causes an increasing perturbation of the bare H+Wn cluster with size by symmetry reduction and the strong OH···π H-bonds. Comparison of H+(Np-Wn) with the related H+(Bz-Wn) clusters (Bz = benzene) indicates the implications of extending the aromatic π-electron system on both the critical threshold size for ICPT (nc = 1 for Bz and nc = 2 for Np) and the structure of the hydration network.